1. Technical Field
This invention relates generally to developer mixture monitors for detecting the concentration of toner particles in electrostatographic machines, and more particularly to such monitors adapted to best cooperate with development stations having curved sump walls.
2. Background Art
Known magnetic toner monitors rely on a sensing element which consists of an electromagnet solenoid having a coil-form with one end adapted to be directed toward the development mixture to be monitored. FIG. 1 illustrates a typical toner monitor 10, which includes an unshielded solenoid winding 12 wound on a coil-form 14. Some monitors include a shielding cup 16 of permeable material for confining the magnetic field path to reduce the effect on the solenoid's inductance of material behind and beside the solenoid. The monitor is generally enclosed in a plastic protective covering 18. The face of the protective covering abuts the outer surface of the development station sump wall 20, or, alternatively, protrudes through a hole in the development station wall.
Such monitors produce a signal characteristic of changes to the inductance of the solenoid due to the presence of magnetic material within the fringe field of the solenoid. Since toner particles are formed of magnetic material and carrier is not, the signal produced by the monitor is proportional to the concentration of toner particles in the portion of the development mixture that is within the fringe field of the solenoid. The more of the fringe field that is filled with the development mixture, the greater will be the monitor's sensitivity.
At the open of these conventional shielded toner monitors, solenoid winding 12 and surrounding shielding cup 16 terminate in a single plane, even with one another; for they are intended to be mounted against the flat, planar outer surface of sump wall 20, as illustrated in FIG. 1; or to protrude through the sump wall. Area 22 of FIG. 1 generally illustrates a volume comprising the region of the sump where the monitor is sensitive to development material. The monitor would, of course, be sensitive to material between area 22 and shielding cup 16 except that this region comprises the protective covering 18 and possibly the station wall 20.
Referring to FIG. 2, current toning station technology has largely eliminated the classical sump. Newer stations feature sump walls with curved inner surfaces which closely follow the radius of an internal paddle-wheel, ribbon blender, or similarly-shaped longitudinal mixer/blender. The outer surface of the sump wall may be flat, as shown in FIG. 2, or curved as shown in commonly assigned U.S. Pat. No.4,775,915, which issued to George R. Walgrove on Oct. 4, 1988.
If the radius of such a curved sump inner wall surface is large, then flat-faced traditional monitors can be used without severe problems. In more severe situations of shorter-radius inner wall configurations, a substantially larger portion of the sensitive volume 22 is occupied by the sump wall, as indicated in FIG. 2 by the area above the dotted lines; leaving less of the development mixture in the sensitive volume. This decreases the sensitivity of the toner monitor.